Carbon nanotubes (CNTS) can be described as graphene sheets rolled into seamless cylindrical shapes (Ebbesen 1994; Poole and Owens 2003) with such a small diameter that the aspect ratio (length/diameter) is large enough to become a one-dimensional structure in terms of electronic transmission. Since its discovery in 1991 by Iijima while experimenting on fullerene and looking into soot residues, two types of nanotubes have been made, which are single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). SWNT consist only of a single graphene sheet with one atomic layer in thickness, while MWNT is formed from two to several tens of graphene sheets arranged concentrically into tube structures. The SWNTs have three basic geometries, armchair, zigzag and chiral forms. They are promising one-dimensional (1-D) periodic structure along the axis of the tube, where dispersion relations predict interesting electronic characteristics. Electronic transport confinement in the radial direction is maintained by the monolayer thickness of the nanotubes, while circumferentially, periodic boundary conditions are imposed leading into 1-D dispersion relations for electrons and photons in SWNTs. Various experiments and simulations have estimated that about one-third of the nanotubes are metallic and two-third semiconducting as a result of the tube diameter and the chiral angle between its axis and the zigzag direction. Quite a number of resistivity measurements on MWNTs have revealed that some are metallic as much as graphite, and others are semiconducting having resistivity value in the order of 5 greater than the former. Experiments had affirmed that armchair carbon nanotubes are metallic (Ebbesen 1997). Multi-walled nanotubes, on the other hand, have not only shown metallic, semiconducting and mechanical properties, they are also being used for fuel storage such as hydrogen and methane (Iyuke 2001, Iyuke et al. 2004). It is, therefore, interesting to infer that CNT is emerging as a building block for nanotechnology, nanoelectronics, nanomanufacturing and nanofabrication.
The growth of CNTs during synthesis and production is believed to commence from the recombination of carbon atoms split by heat from its precursor. Due to the overwhelming interest, enormous progress is being made in the synthesis of CNTs. Although a number of newer production techniques are being developed, the three main methods are laser ablation, electric arc discharge and chemical vapour deposition (CVD). The last is becoming very popular because of its potential for scale-up production. The existing methods involve batch operation that can only produce maximum amount of 0.5-5 g per day and poor products repeatability. Due to the low production rate, it has an exorbitant cost at US$500 per gram.
CVD methodology in producing CNTs bears little difference to the conventional vapour grown carbon fibre technology. As the latter technology improved into producing thinner carbon fibres from several micrometers to less than 100 nm, the growth of carbon nanotubes synthesised using catalytic CVD has emerged. In both cases, carbon fibres and carbon nanotubes may be grown from the decomposition of hydrocarbons in a temperature range of 500 to 1200° C. They can grow on substrates such as carbon, quartz, silicon, etc., or on floating fine catalyst particles, e.g. Fe from ferrocene, Ni, Co, etc. and from numerous hydrocarbons such as benzene, xylene, toluene, natural gas or methane, acetylene, to mention but a few. The CVD method has been receiving continuous improvement since Yacaman et al. first used it in 1993 and, in 1994, Ivanov et al. produced MWNTs using CVD techniques. Patterned silicon wafers of porous n- and p-type types were used to grow regular arrays of MWNTs (Fan et al. 2000), and SMNTs were grown as low and high whisker population density fastened to the filament fibrils for carbon fibre surface treatment (Iyuke et al. 2000). SWNTs produced from floating catalyst CVD was also presented earlier (Fakhru'l, Iyuke and co-workers, 2003). A typical catalytic chemical vapour deposition system consists of a horizontal tubular furnace as the reactor. The tube is made of quartz tube, 30 mm in diameter and 1000 mm in length. Ferrocene and benzene vapours, acting as the catalyst (Fe) and carbon atom precursors respectively, are transported by either argon, hydrogen or a mixture of both into the reaction chamber, and decompose into the respective ions of Fe and carbon atoms, resulting in carbon nanostructures. H2S has also been used as carrier gas and CNT yield promoter. The growth of the nanostructures occurs either in the heating zone, or before or after the heating zone, which is normally operated between 500° C. and 1150° C. for about 30 min. The flow of hydrogen gas is 200 ml/min, while argon gas is used to cool the reactor as reported elsewhere (Danna 2004).
However, for crucially specific applications with CNTs, both the mechanical and electrical properties can be harnessed if long (>2 mm) and continuous CNTs can be produced. Such structures could be used as strong, highly conducting microcables or as robust electrochemical microactuators (Zhu et al 2002). Such a breakthrough is still missing in the open literature.
As indicated above present processes for the production of CNTs are batch processes. These suffer the disadvantages of poor repeatability and poor industrial applicability or efficiency.